This report documents an exploratory research and development program to design, fabricate, and test an engineering demonstration prototype positive-displacement rotary-vaned air cycle machine (Model 606) for the United States Army Mobility Equipment Research and Development Command at Ft. Belvoir, Virginia. Existing ROVAC computer software was used to perform system analysis and to establish machine design parameters. The Model 606 compressor-expander (Circulator) was fabricated using conventional means and special techniques developed by The ROVAC Corporation. The Circulator and the associated air conditioning system evolved through modifications from an open cycle system design to a closed cycle system with regeneration. These changes in the system configuration and modifications in Circulator design were performed to incorporate into the 606 breadboard system all promising improvements which are continuously derived from ROVAC's internally funded advanced development programs.